Walking the Wall

There’s a fungus among us

It was a bit disturbing. There were black, flaking growths all along the face of the wall; dirty white pimples, fluorescent lime pustules, greenish warts. Should we be worried? Did our beloved Great Wall have a deadly disease?

Quite the opposite as it turned out, to our relief. A quick inquiry with Dr David Eldridge, of the University of New South Wales in Sydney, revealed that these are biological soil crusts, an entirely benign phenomenon and even an important component of soil health in arid and semi-arid environments.

Biological soil crusts are formed by living organisms that bind together particles of soils into a crust. They can be made up of cyanobacteria (blue-green algae), mosses and lichens (composite plants consisting of fungi living symbiotically with algae). Biological soil crusts play an important role fixing nitrogen in the soil, improving soil’s resistance to wind and water erosion, and contributing to plant growth, but they are highly vulnerable to damage from livestock.

On the environment provided by the wall, however, biological crusts are very extensive and they’re in great shape, as it’s a bit hard for the sheep and goats to trample a vertical surface. The crusts are also quite pretty, almost like the coral of the desert (okay, it’s no Barrier Reef, but still). And like coral, they make a barrier, in this case the man-made barrier that is the Great Wall of China, come alive.

To draw one final parallel with coral, biological soil crusts come in endless variety. Luckily for us, we have one of Australia’s leading authorities on biological soil crusts on hand to provide tentative identifications. A big thank you to Dr David Eldridge for lending us his expertise!

The photos below are images of biological soil crusts and are excellent examples of the types of species that grow in highly calcareous (high pH) soil and in areas where gypsum is common in the soil.

The black lichen in this photo is a good example of a pioneering lichens called Collema coccophorum. It is a primitive (unstratified) lichen; i.e. the algal and fungal cells are mixed up in the lichen rather than being in layers. It fixes atmospheric nitrogen and makes it available to growing plants; a good thing in an environment where nutrients are limited.

Collema coccophorum is also the lichen that forms the dark patches evident in this photo of the wall . . . the aspect here is probably important; crusts like to be fully exposed to the light, and here there are no plants to shade them so they are able to dominate the exposed face of the wall and give it a lot of protection against wind and rain. The photo at the top of this post is more Collema, with a rosette of probably Caloplaca or Candelariella. The colours in the lichen are due to acids which make them more resistant to UV light.

This is a very dry looking moss, most likely Didymodon torquatus. It survives in a hostile environment by curling its leaves up against its stem. This reduces the surface area and prevents it from drying out. When it rains, this and many other similar mosses rapidly uncoil and begin photosynthesising.

Here we have a classic squamulose (warty) lichen of desert areas, Placidium squamulosum. Notice how the lichen forms little cracks and nooks in the soil surface. These are good niches for seeds to germinate in.

This is a rich community of squamulose Placidium squamulosum and Psora crystallifera (right at the bottom). This Psora has a pyramidal surface which helps it to diffuse light and reduce its surface temperature. Water also collects in the grooves on the lichen surface and may advantage cyanobacteria which grow in soil along the edge of the lichen. The greenish-whitish-yellow lichen is Xanthoparmelia, a foliose (leafy) lichen. It is very susceptible to excessive trampling. It relies on a small amount of fragmentation to establish in new areas, but too many hooves are deadly to this lichen. It is very slow growing.

The material in block quotes was provided by Dr David Eldridge of the School of Biological, Earth & Environmental Sciences at the University of New South Wales.